jsonschema: support formats

[jba]

The format keyword is a string that describes the kind of value that the schema represents, like "date" or "email". See the spec.

The Schema type contains a Format field, but does nothing with it. Depending on other settings, as described in the spec, it could be used to validate or just "annotate."

This package doesn't support annotations except as an internal bookkeeping mechanism for validation, and I'm inclined to keep it that way unless there is demand.

We could validate against formats. Best practice advises against that, recommending the pattern keyword instead, but some uses may require it.

[jba]

I am going to close this, because as I argued in the top post, it goes against best practices.